Higher performance, lower cost, increased miniaturization of components, and greater packing density of integrated circuits are ongoing goals of the computer industry. Greater integrated circuit density is primarily limited by the space or “real estate” available for mounting microelectronic components on a substrate such as a printed circuit board. The microelectronic component may be electrically connected to circuitry on the circuit board via flip chip attachments, wirebonding, tape automated bonding (TAB), or a variety of other techniques.
Increasingly, microelectronic components are being vertically stacked atop one another to conserve valuable substrate real estate. In such a vertically stacked assembly, a first microelectronic component is attached directly to the substrate and a second microelectronic component may be physically attached to the first microelectronic component (e.g., stacked on the first microelectronic component). If the first microelectronic component is electrically connected to the substrate via flip chip attachments or TAB, the active surface of the microelectronic component (i.e., the surface bearing the electrical contacts for connection to the circuitry of the microelectronic component) faces toward the substrate. Commonly, the bare backside surface of the first microelectronic component is exposed and faces away from the substrate, and the second microelectronic component is attached directly to the backside surface.
If the first microelectronic component is electrically connected to the substrate by wire bonding, however, attachment of the second microelectronic component to the first microelectronic component can be more problematic. In wire-bonding techniques, the backside of the first microelectronic component is mounted to the substrate and the active surface of a wire-bonded microelectronic component defines the outer surface which faces away from the substrate. The contacts on the active surface are then electrically coupled to the contacts on the substrate by very small conductive wires that extend from the active surface to the substrate. The wires that electrically connect the active surface of the microelectronic component to the substrate accordingly interfere with attaching the second microelectronic component directly on the active surface. FIGS. 1 and 2 schematically illustrate two techniques currently used to bond a second microelectronic device to a first microelectronic device which is wire-bonded to the substrate.
FIG. 1 illustrates a substrate 20 carrying a pair of microelectronic devices 30, 40. The substrate 20, which may be a circuit board or the like, has a contact surface 24 bearing a plurality of electrical contacts 26a–26d. A first microelectronic component 30 is attached to the component surface 24 of the substrate 20 by means of an adhesive 35. The adhesive 35 may cover the entire mounting face 32 of the first microelectronic component 30. The active surface 34 to the first microelectronic component 30 includes a plurality of electrical contacts 36a–36b. A first bonding wire 38a electrically connects the first electrical contact 36a of the first microelectronic component 30 to the first electrical contact 26a of the substrate 20, and a second bonding wire 38b electrically connects a second electrical contact 36b of the first microelectronic component 30 to a second electrical contact 26b of the substrate 20.
The second microelectronic component 40 is carried by the first microelectronic component 30. In some conventional stacked microelectronic devices, a facing surface 42 of the second microelectronic component is attached to the active surface 34 of the first microelectronic component 30 via a single, thick adhesive layer (not shown). This adhesive layer conventionally has a thickness which is greater than the height to which the bonding wires 38 extend above the active surface 34 so the second microelectronic device 40 does not directly contact or rest against the bonding wires 38. Such a structure is shown in U.S. Pat. No. 5,323,060, the entirety of which is incorporated herein by reference. In the embodiment shown in FIG. 1, a separate spacer 50 is positioned between the first and second microelectronic components 30 and 40. This spacer 50 is attached to the active surface 34 of the first microelectronic component 30 via one adhesive layer 52 and is attached to the facing surface 42 of the second microelectronic component 40 by another adhesive layer 54. The spacer 50 is commonly either a polymeric tape or a thin silicon wafer. Once the second microelectronic device 40 is in place, a first electrical contact 46a on the outer surface 44 of the second microelectronic component 40 can be electrically connected to a third electrical contact 26c carried by the substrate 20. Similarly, a second electrical contact 46b on the outer surface 44 can be electrically connected to a fourth electrical contact 26d carried by the substrate 20.
A stacked microelectronic device such as that shown in FIG. 1 can present some manufacturing difficulties. For example, rapidly and precisely positioning the spacer 50 and adhesive layers 52, 54 can be a challenge. Even if the stacked microelectronic device is properly assembled initially, the multiple layers of different materials can lead to product defects. If the second microelectronic component 40 is attached to the first microelectronic component 30 by a single, thick adhesive layer (not shown), any difference in the coefficient of thermal expansion between the adhesive layer and the microelectronic components 30 and 40 can cause deleterious warping of the microelectronic components 30 and 40. If a polymeric tape is used as the spacer 50 shown in FIG. 1, differences in the coefficients of thermal expansion can still lead to warping of the microelectronic components 30 and 40 during subsequent thermal processing. If the microelectronic components 30 and 40 are silicon-based dies, the use of a silicon spacer 50 can reduce the problems associated with differences in coefficients of thermal expansion. However, the different coefficient of thermal expansion of the adhesive layers 52 and 54 can still induce some stress in the microelectronic components 30 and 40. In addition, the structure shown in FIG. 1 has four separate interfaces between the first and second microelectronic components 30 and 40, namely the interface between the active surface 34 of the first microelectronic component 30 and the adhesive layer 52; the interface between the adhesive layer 52 and the spacer 50; the interface between the spacer 50 and the adhesive layer 54; and the interface between the adhesive layer 54 and the facing surface 42 of the second microelectronic component 40. Each additional interface increases the number of manufacturing steps, necessitating more manufacturing time and/or equipment, and heightens the risk of producing an unacceptable stacked microelectronic device. Spacers also increase the size of the final stacked microelectronic device, with both silicon and polymer spacers having typical thicknesses on the order of 6 mils.
FIG. 2 schematically illustrates another conventional stacked microelectronic device. Instead of having a thick adhesive layer or a spacer 50 between the first and second microelectronic components 30 and 40, the second microelectronic component 40 may be attached to the active surface 34 of the first microelectronic component 30 is using a relatively thin adhesive layer 55. To avoid interference with the bonding wires 38, however, the second microelectronic component 40 is significantly smaller than the first microelectronic component 30. While this avoids direct physical contact between the second microelectronic component 40 and the bonding wires 38, it significantly limits the size of the second microelectronic component 40. Although the stacked microelectronic device of FIG. 2 can be made shorter than the stacked device of FIG. 1, the smaller size of the second microelectronic component 40 reduces the number of integrated circuits which can be incorporated in the second microelectronic component 40 and, hence, the stacked microelectronic device.